HTTP status code

The HTTP 409 status code stands for "Conflict". It is a client error response indicating that the request could not be completed because it conflicts with the current state of the server or resource. This conflict is typically due to a concurrent modification or state inconsistency.

When is HTTP 409 Used?

• When there is a conflict between the request and the current state of the resource.
• Commonly seen in version control systems, database operations, or API calls where a resource is simultaneously modified.
• When a duplicate resource is attempted to be created, or there is a conflict in data (e.g., updating a resource with outdated or conflicting information).

Example Scenarios

• Concurrent Modification: Two clients try to update the same resource at the same time, causing a conflict.
• Version Conflict: A client attempts to update a resource using an outdated version (e.g., submitting a change with an old resource ID).
• Duplicate Resource Creation: Trying to create a resource (like a user account) that already exists.

How Does It Work?

The server returns a 409 Conflict response to indicate that the current request cannot be processed due to conflicts with the existing state or resource. The response may include details on how to resolve the conflict.

Example

Client Request (Trying to Update a Resource with an Outdated Version)

    
        PUT /users/123 HTTP/1.1
        Host: example.com
        If-Match: "v1"
        Content-Type: application/json
        {
          "name": "John Doe",
          "email": "john.doe@example.com"
        }
    

(The client attempts to update the user profile, but the version provided in If-Match is outdated, and a newer version exists.)

Server Response (409 Conflict Due to Version Mismatch)

    
        HTTP/1.1 409 Conflict
        Content-Type: application/json

        {
          "error": "Conflict: The version of the resource you are trying to update has been modified. Please fetch the latest version and try again."
        }
    

Best Practices for Handling HTTP 409

• Use Optimistic Concurrency Control: Ensure that clients work with up-to-date data to avoid version conflicts.
• Provide Clear Error Messages: Include specific details in the response to help the client resolve the conflict (e.g., a link to the latest version of the resource).
• Prevent Duplicate Resources: Implement checks to avoid creating duplicate records that may lead to conflicts (e.g., unique constraints for email addresses or usernames).
• Retry Logic: In cases of concurrent modifications, prompt the user to retry their request after reviewing the latest resource state.

Summary

HTTP 409 indicates that the request cannot be processed because it conflicts with the current state of the resource, often due to concurrent changes, outdated data, or duplicate entries. This status code is commonly used in APIs and versioned systems to handle conflicts that need resolution before proceeding.